The usual apparatus for pumping well fluids from a well to the surface through a tubing, which extends from the surface to a fluid producing formation penetrated by the well, includes a pump connected to the lower end of the tubing which has a plunger or "traveling valve" which is reciprocated in the longitudinal barrel of the pump by a string of sucker rods, the bottom rod being connected to the traveling valve, the top rod of the rod string being connected to a motor driven means for alternately pulling the string upwardly and then allowing the string to be moved downwardly by gravity.
Since the weight of the pump rod string provides the force necessary to cause well fluids to flow upwardly through the tubing, if resistance to the downward movement of string of tubing by the upwardly flowing well fluids is great, especially past such obstructions to fluid flow as rod guides or scrapers mounted on the rods, the rod string will move downwardly relatively slowly thus reducing the rate of production of the well fluids. The rod guides of course decrease the flow space area between the rod and the tubing having radially outwardly extending longitudinal ribs for engaging the internal surfaces of the well tubing to limit the lateral displacement of the rod in the tubing. In addition, if the pump and lower end portions of the rod string offer a relatively great resistance to downward movement of the rod string, the weight of upper portions of the rod string may cause lower portions of the rod string to be placed under great compression loads which tend to cause such lower portions to bend and buckle and their rod guides to be moved lateral and pressed with great force against the internal surfaces of the tubing.
The rod guides at locations where the rod is not centrally positioned in the well tubing rub against the internal surfaces of the well tubing over the distance, for example, six feet, of the reciprocal movement of the sucker rod string during pumping operation. The rod guides accordingly must be formed of a substance which is hard and durable, but which does not have such abrasive characteristics as to abrade or erode the well tubing to such an extent that the well tubing is damaged and even broken. The substance of which the rod guides are formed must also be durable so that the rod guides themselves are not worn away or eroded in relatively short time period of the reciprocating operation of the rod string.
Since the well tubing itself may not be perfectly vertical and be bent at various locations along its length the rod guides must limit the lateral displacement of the rod relative to the central longitudinal axis of the well tubing at such locations. The ribs of the rod guides at locations of deviated or bent portions of the tubing or of the rod string will abrade or erode since they slide in frictional engagement with the internal surfaces of the well tubing. The two ribs of the rod guide which engage the well tubing at such locations would be worn away quickly and allow portions of the string, as at the locations of the upsets at the ends of each rod by which the rods are connected to one another, actually to come in metal-to-metal contact with the internal surfaces of the tubing and so damage the tubing as well as the rods themselves.
In many installations the well head equipment at the surface of the well includes a rotator which rotates the rod string a few degrees in a step by step manner so that all ribs are sequentially moved into contact with the internal surfaces of the well tubing at such locations of the curvature or lateral displacement of the tubing or the rod string so as to lengthen the time before the failure of the rod guide due to excessive wear of its ribs.
At some locations of lateral displacement of the rod guide string relative to the longitudinal axis of the tubing, the two guide ribs which engage the internal surfaces of the tubing to limit such lateral displacement may be held with such great force against the internal surface of the well tubing that the portions of the rod string between such rotator and such rod guide may actually twist 360 degrees or more, especially if the length of such portions are of great length, a thousand or more feet, before sufficient torque force is exerted on the rod to cause the rod at such location to be rotated until the violently and rapidly engagement of another rib with the well tubing again limits such lateral displacement and rotation. Once the rotation of the rod has started, the rod may rotate rapidly relative to the tubing with consequent damage to the tubing and the ribs which engage it during such violent rotation of the rod string.
It is also desirable that in one form the rod guide of the invention have means for ensuring that during the rotation of the rod string at least two ribs radially spaced ninety degrees apart be continuously in engagement with the well tubing at the locations of lateral displacement of the rod string relative to the well tubing in order to prevent abrupt rotational movement of the rod as a pair of the ribs moves out of engagement with the tubing before another pair engages the tubing since abrupt jerking rotation of the rod string may actually cause damage to the rod guides and to the well tubing.
The rod guides also must not cause excessive torque forces to be exerted on the rods at such locations since the tubing is circular and tends to force or torque one rib in one direction about the longitudinal axis of the rod shank so that a second rib of the rod guide must be so positioned and engaged with the internal surface of the well tubing as to provide a torque force acting in the opposite direction so that the rod string itself is not rotated or twisted by the great forces exerted on the rod guide.
In addition, the rod guides should provide a relatively low resistence to upward flow of well fluids therepast and therefore to the downward movement of the rod string in the tubing as has been disclosed in the U.S. Pat. Nos. 4,809,777 and 4,997,039 to Donald E. Sable. The rod guides should also minimize the turbulence in the well fluids flowing therepast during the reciprocatory movement of the rod string in the well tubing which not only increases the resistence to upwardly flow of the well fluids, but also tends to wear away and damage the rods of the rod string and the internal surfaces of the well tubing, specially if the well fluids include abrasive particles, such as sand, or corrosive chemicals.
The plastic substances available and suitable for use to form the rod guides are hard and durable, but somewhat brittle so that the length of the bodies of the rod guides which regidify the longitudinal portions of the rod shank on which they are rigidly mounted are limited because the rods are pressured to bend laterally thus cracking the bodies of the rod guide.
The ribs of the rod guides which engage the internal surfaces of the well tubing should preferably provide a fairly large area of contact with the well tubing at locations of their sliding engagement therewith in order to minimize the per unit area pressure or force with which the ribs are forced into engagement with the well tubing, both to minimize wear of the ribs and the wear and abrasion of the well tubing itself.
While the rod guides disclosed in the U.S. Pat. No. 4,997,039 to Donald E. Sable minimizes the damage to the elongate rod guide body caused by the bending of the rod by providing a circular area of low mechanical strength and decreased radius in order to cause any such damage or cracking of the rod guide to be limited to a central area between two longitudinally spaced pairs of guide ribs, the cracking of the body at such locations of the rod guide body exposes the rod at the locations of such cracks to the well fluids and increases the turbulence thereof at such locations. The rods are cleaned of all dirt and coating substances in order to prevent outgasing therefrom and provide great adherence of the guide bodies to the rod shanks. Since the rod is not coated with any corrosion inhibiting material beneath the rod guides, such exposed portions of the rod are more susceptible to damage by turbulence, chemical action and abrasion by abrasive particles in the well fluids.
It is desirable therefore that each rod guide be provided with two pairs of longitudinally spaced units each having a body provided with a pair of diametrically outwardly extending radial ribs, the ribs of one unit being displaced ninety degrees about the longitudinal axis of the rod shank from the ribs of the other unit and spaced on the rod shank so that a flex portion of the rod shank between the two units of the rod guide be free to accommodate any lateral forces exerted thereon, the two bodies of course regidifying the portions of the rod shank above and below such flex portion.
It is also desired that rod guides having such two separate units be installable or molded simultaneously on a rod shank in order to achieve economies in time of manufacture, the molding process requiring the injection of hot molten plastic into cavities of the molding apparatus which then must be cooled sufficiently before the molding equipment can again be used to form another rod guide on the same shank and spaced from the previously molded rod guide on the same rod shank by some distance, for example, six feet or more.